Tower crane with load limiting apparatus

ABSTRACT

The load limiting apparatus of a tower crane, which inactivates the load lifting winch when a predetermined load is exceeded, is controlled in such a manner that the maximum permissible load Lmax varies with the angle Alpha between the boom and the horizontal, and the corresponding horizontal spacing a between the projections of the boom pivot and of the depending load carrying cable in a manner to satisfy the equation Lmax L0max k1a wherein L0max is the value of Lmax when a is at a minimum, and k1 is a proportionality factor and constant for all practical purposes. Under these conditions, the maximum tensile stress developed in the boom is constant, at least for an optimum boom length, while a varies between amin and amax, and the structural strength of the material can be fully utilized under all conditions.

United States Patent I [1 1 Tax et al.

[ Oct. 23, 1973 TOWER CRANE WITH LOAD LIMITING APPARATUS [73] Assignee:said Tax, by said Franke & said Hosler [22] Filed: Jan. 25, 1972 [21]Appl. No.: 220,609

[30] Foreign Application Priority Data Feb. 2, 1971 Germany P 21 04800.0

[56} Relerences Cited UNITED STATES PATENTS 3,072,264 l/l963 Sennebogen212/39 MS 3,037,641 6/1962 Potter et al. 212/144 X FOREIGN PATENTS ORAPPLICATIONS 2/l96l Germany 212/39 DB 1,060,089 2/l967 Great Britain212/39 R Primary ExaminerEdward A. Sroka Attorney-Kurt Kelman et al.

[57] ABSTRACT The load limiting apparatus of a tower crane, whichinactivates the load lifting winch when a predetermined load isexceeded, is controlled in such a manner that the maximum permissibleload L,,,,, varies with the angle a between the boom and the horizontal,and the corresponding horizontal spacing a between the projections ofthe boom pivot and of the depending load carrying cable in a manner tosatisfy the equation L,,,,,, L k,a wherein L is thevalue of L when a isat a minimum, and k is a proportionality factor and constant for allpractical purposes. Under these conditions the maximum tensile stressdeveloped in the boom is constant, at least for an optimum boom length,while a varies between a,,,,,, and a and the structural strength of thematerial can be fully utilized under all conditions.

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TOWER CRANE WITH LOAD LIMITING APPARATUS BACKGROUND OF THE INVENTIONThis invention relates to tower cranes, and particularly to tower cranesequipped with an improved load limiting apparatus.

The stresses in the boom of a tower crane at a fixed load vary as thehorizontal spacing between the tower and the load varies, that is, asthe boom is raised and lowered. To avoid failure of the boom, knowntower cranes are equipped with load limiting apparatus which inactivatesthe load lifting winch or winches when a load applied to a load liftingtension member, such as a cable, rope, or chain, exceeds a value L,,,,,which varies with the horizontal spacing a of the load or of thedepending part of the load lifting tension member from the tower, morespecifically from the pivot of the boom, as determined by the equationaL constant This equation has its root in the historical development ofcrane construction. The term aL was used in the early days of towercrane construction as a guide in the selection of a counterweight whichwas preferably made one-half aL for best stability of the crane.

With the increasing height of tower cranes, the load moment aL is nolonger the primary factor determining the stability of the crane becauseit does not reflect the effects of wind pressure. Modern tower cranesare therefore equipped with central ballast sufficient to absorb theforces exerted by wind and by the load under all usual conditions, yet,the load limiting devices employed heretofore were still designed tomake the permissible maximum load inversely proportional to thehorizontal distance between the load and the crane tower.

It has now been found that the conventional load limiting devices do notpermit loading of a tower crane to the full design capacity of itsstructure, and it is a primary object of this invention to permit largerloads to be safely lifted by tower cranes without increasing the weightand cost of its structure.

SUMMARY OF THE INVENTION According to this invention, a tower crane maysafely be loaded to a maximum load L,,,',;, determined by the equation,...r L4 i wherein L is the permissible maximum load when a is a,,,,,,that is, the spacing between the load and the pivot of the boom,projected into a common horizontal plane, is at the smallest availablevalue, and k is proportionality factor which is practically constant.Within limits set by the deviation of k, from a perfectly constantvalue, the equation presented above is that of a straight line, whereasthe equation aL,,,,, constant is that of a curve.

The higher permissible load in a crane equipped with a load limitingdevice of the invention has been found not to affect the stability of aconventional tower crane with its central ballast.

It has furthermore been found that a load limiting device whichinactivates the load lifting winch when L,,,,,,

exceeds the value determined by the above equation causes the maximumstress in the crane boom to be practically uniform for all values of afor at least one specific boom length readily ascertained bycalculation, and that the stresses in the boom under the permissiblemaximum load are held near the design stress at all angles ofinclination of the'boom, thereby permitting the structural strength ofthe boom to be utilized to its fullest advantage. The boom employed ispreferably loaded in compression only and free from bending stressesother than those caused by its own weight.

Other features and objects of the invention will presently becomeapparent.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 shows a tower crane equippedwith the load limiting device of the invention;

FIG. 2 shows a portion of the tower crane of FIG. 1 including the loadlimiting device of the invention.

FIG. 3 is a diagram illustrating the relationship of the maximumpermissible load L and the horizontal spacing a of the load from thetower for'booms of different length in a tower crane equipped with aconventional load limiting device;

FIG. 4 illustrates the relationship of permissible load L and thehorizontal spacing of the load from the tower for a tower crane equippedwith a protection device of the invention, but otherwise analogous tothe crane referred to in FIG. 3;

FIG. 5 shows the improvement of the load bearing capacity of the craneof the invention as compared to an otherwise analogous crane as afunction of the angle of inclination of two booms of different length;

FIG. 6 illustrates the maximum tensile stresses in booms of differentlength in a crane equipped with the conventional load limiting device asa function of the inclination of the boom; and

FIG. 7 shows the corresponding maximum tensile stresses in a craneaccording to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The tower crane of the inventionillustrated in FIG. 1 has'a base structure 10 including a vehicularlower part 12 whose center portion 14 provides a bearing for a turntable16. The tower proper 18 of the crane is mounted on the'turntable l6 andis a vertical column having an obliquely inclined head portion 20. Theboom 22 of the crane is a compression member, not subjected tosignificant bending stresses, and one of its ends is secured to theupper end of the column 18 by a pivot 24.

The boom 22 is angularly adjusted on the pivot 24 by a cable or cablesystem 26 which is operated by a winch 28 on the turntable 1 6 and,passing over guide pulleys 30, 34 on a rearwardly extending jib 32 andon the head portion 20, it is attached to the boom 22 near its front orfree end by a suitable fastener 36. One end of the jib 32 is suspendedfrom the head portion 20 by a cable 33.

A load may be raised or lowered by a lifting cable 38 operated by awinch 40 on the turntable 16. The cable 38 is trained over guide pulleys42, 44, 46 respectively mounted on the jib 32, the head portion 20, andthe front end of the boom 22, and a reversing pulley on a hook 48 to befastened to an attachment 50 on the boom 22 near the free end of thelatter.

The tower column 18 has upright front and rear frames 52, 54 which areconnected by oblique braces 53 and by horizontal struts 3 of which one,of U-shaped cross section, is shown in FIG. 2. The end of the jib 32remote from the cable 33 is fastened to the illustrated strut 3 by apivot 4. The guide pulley 42 is mounted on a bracket 56 attached to thejib 32 by a pivot pin 58. A switch actuating abutment 60 is mounted onthe bracket 56 adjacenta switch 62 arranged on a post 63 which isfixedly mounted on the jib 32.

The abutment 60 is normally held out of engagement with the switch 62,as shown in FIG. 2, by a helical compression spring 64 coiled about apin 68. The two axial ends of the spring 64 respectively abut against aguide sleeve 66 pivotally mounted on the bracket 56 and slidablyreceiving the pin 68, and against a collar 70 on the pin'68. A rod 72coaxially attached to the pin 68 is secured to the boom 22 by a pivotpin 74. The pivot pin which attaches the sleeve 66 to the bracket 56 maybe shifted between several bearing openings in the bracket 56 foradapting the load limiting device to dif ferent boom lengths.

The tension in the load lifting cable 38 tends to pivot the bracket 56clockwise as viewed in FIG. 2. When the load is sufficient to overcomethe restraint of the spring 64, the switch 62 operates a relay circuitin a control box 76 in a conventional manner and stops the winch 40. Aload exceeding the maximum load determined by the limiting device thuscannot be lifted.

The operating characteristics of the load limiting device of thisinvention will be described hereinafter with reference to dimensionsindicated in FIGS. 1 and 2, wherein h is the distance between the pivotaxes of the pivot pins 24, 74 engaging the boom 22, and a is the angleof inclination of the elongated boom 22 relative to the horizontal.

FIG. 3 illustrates the relationship between the maximum permissible loadL and the effective horizontal spacing a of 'the'load from a crane towerequipped with conventional load limiting devices. The full length of theboom is assumed to be 45 meters, and separate curves are shown foroperation at full boom length (100 percent) and with booms shortened insteps of one-eighth to 87.5 percent, 75 percent, and 62.5 percentrespectively. a

The abscissa indicates the distance a (see FIG. 1) between the pivot 24and the load hook 48 projected into a horizontal plane, and it variesfor a boom of a given length with the angle a of boom inclination. Theordinate is calibrated in metric tons. The curves representing therelationship between maximal permissible load and horizontal spacing ofthe load from the pivot 24 of the boom 22 on the tower 18 are curveswhose concave faces are directed toward higher loads. They arecharacteristic of tower cranes equipped with conventional load limitingdevices.

When the same tower crane was equipped with the load limiting device ofthe invention described with reference to FIG. 2, and the relationshipof maximum permissible load and effective horizontal spacing of load andtower was again determined and charted in the manner of FIG. 3, thegraph of FIG. 4 was obtained in which the relationship between maximumload and effective horizontal load spacing is practically linear forbooms of all tested lengths.

The lines shown in FIG. 4 satisfy the equation ina: ona: l

wherein L is the maximum load capacity to be determined, and L is themaximum load permissible for minimum effective horizontal spacing a. Inthe crane chosen as an example, the smallest available, horizontalspacing a of the hook 48 and the pivot 24 is 12.9 meters; k is aconstant proportinality factor.

Because of the relationship of the load spacing, the angle a of boominclination, and the effective length I of the boom, equation (I) may berewritten as mnr orna: 1 cos a The force exerted on the guide pulley 42by the load lifting cable 38 is directly proportional to the loadsupported by the cable. At maximum permissible load L the correspondingeffective force R exerted by the cable on the pulley reaches the valueand I k L k k lcos a (IV) By combining the constants in equation (IV),there is obtained the following relationship for the cable force exertedon the guide pulley 42:

R A k,cos a To achieve the several linear relationships illustrated inFIG. 4, it is necessary that the stress V in the helical compressionspring 64 vary with a in the same manner as the force exerted by thecable 38 on the pulley 42:

I V A k cos a I That the device shown in FIG. 2 can satisfy this condi-'tion will now be shown. The stress of the helical compression spring 64varies according to the equation V V, k hcos a V B k,cos a (VIII) when His chosen equal to A in equation (VI) and k, is chosen to be equal tok,, the relationship of L to it becomes linear, as illustrated in FIG.4.

FIG. 5 illustrates the percentage increase in load bearing capacity as afunction of the angle of boom inclination when the conventional loadlimiting device is replaced by that of the invention. FIG. 5 has beenderived from the values of FIGS. 3 and 4 for efi'ective boom lengths of100 percent and 87.5 percent. It is evident that the maximum permissibleload L is greatly increased for values of a which are intermediate themaximum and minimum.

FIG. 6 illustrates the variation in the maximum tensile stress in theboom 22 when the crane is equipped with the known load limiting devicewhich is characterized by the curves of FIG. 3. The actual stresses arewell below the permissible stress in the curve illustrating thecondition of the boom of full length (100 percent), and the stresses areeven lower in shorter booms, the permissible stress having a value of2,400 kp/mm in this instance.

FIG. 7 shows the maximum tensile stresses generated in booms ofdifferent lengths with the load limiting device of the invention whosecharacteristics are illustrated by FIG. 4. The actual stresses generatedin the boom of 100 percent length when the crane is operated at the fullload tolerated by the limiting device differ from the design load overthe entire range of boom inclination by not more than 5 percent so thatthe material of the boom is utilized to best advantage.

In the preceding discussion of numerical relationships, k was consideredto be absolutely constant, yet, many of the advantages of this inventionare available when k, is at least substantially constant, but varies dueto secondary factors, not considered above, to let the values of Ldiffer by not more than 10 percent, and preferably not more than 5percent from the values of L defined by Equation (I) when k isabsolutely constant while a varies between a the horizontal spacing ofthe lifting cable 38 and the pivot 34 when the boom is as close to ahorizontal position as the crane permits, and a the horizontal spacingof the cable 38 and the pivot 24 when the boom is as close to a verticalposition as is possible.

Similar considerations hold for the maximum tensile stress in the boom22. The material of construction of the boom (and of the column 18) isused to best advantage if the load carried by the cable 38 is limited insuch a manner that the maximum tensile stress at permitted maximalloading of the cable is precisely constant and equal to the maximumdesign load. It would not be economical under most conditions to designa tower crane to such a precise performance, and the major advantages ofthis invention are available if the tensile stress in the boom differsby not more than plus/minus 10 percent, and preferably plus/minus 5percent from a constant value while the spacing a varies between a and av I The load limiting device of the invention illustrated in FIG. 2 is,in effect, a mechanical comparator. The stress V in the spring 64 isgenerated in response to the angle of inclination a of the boom, or thespacing a between the pivot 24 and the depending cable 38, as projectedinto a horizontal plane. The cable 38 and the pulley 42 generate anotherforce R in response to the magnitude of the load supported by the cable.The two forces V and R are both applied to the bracket 56 in such amanner as to oppose each other, and the bracket transmits the twoforces, or their resultant force to the switch actuating abutment 60which inactivates the winch 40 when the relationship of the forces V andR exceeds a predetermined value set, at least in part, by shifting thepivot axis of the bracket 56.

It will be appreciated that the mechanical comparator illustrated inFIG. 2 may be replaced by many anal-' ogous mechanical comparators or bytheir electrical and electronic equivalents.

It should be understood, therefore, that the foregoing disclosurerelates only to a preferred embodiment of the invention, and that it isintended to cover all changes and mofifications of the example of theinvention herein chosen for the purpose of the disclosure which do notconstitute departures from the spirit and scope of the invention setforth in the appended claims.

What is claimed is: g

l. A tower crane comprising:

a. a base structure;

b. an upright column upwardly extending from said base structure;

c. a pivot on said column near the upper end of the column;

d. an elongated boom having one longitudinal end portion secured to saidcolumn by said pivot for angular movement in a vertical plane;

e. adjusting means for varying the angular position of said boom in saidplane;

f. a load lifting, elongated tension member depending from the otherlongitudinal end portion of said boom, whereby the spacing of said pivotand of said member projected into a horizontal plane, varies in responseto the variation of said angular position between a minimum value and amaximum value;

g. winch means for longitudinally raising and lowering said tensionmember and a load attached thereto; and a h. load limiting means forinactivating said winch means when a load attached to said tensionmember exceeds a predetermined value,

1. said load limiting means including means for varying saidpredetermined value as a function of said projected spacing during saidvariation of the angular position of said boom to satisfy the equationina: orna: l

wherein L, is predetermined value of said attached load,

a is said projected spacing, L is the value of L, when a a a, beingminimum value of said projected spacing, and k is a proportionalityfactor and substantially constant. v

2. A tower crane as set forth in claim 1, wherein the values of L,differ by not more than 10 percent from the values of L defined by saidequation when k is constant while a varies between a and a a being saidmaximum value.

3. A tower crane as set forth in claim 1, wherein the values of L,differ by not more than 5 percent from the values of L, defined by saidequation when k, is constant while a varies between a and 0. a beingsaid maximum value.

4. A tower crane as set forth in claim 1, wherein the maximum tensilestress in said boom differs by not more than plus/minus 10 percent froma constant value while said a varies between a, and a a being saidmaximum value.

5. A tower crane as set forth in claim 1, wherein the maximum tensilestress in said boom differs by not more than plus/minus percent from aconstant value while said a varies between a,,,,,, and a a being saidmaximum value.

6. A tower crane as set forth in claim 1, wherein said load limitingmeans include means responsive to said spacing a for generating a firstforce, means responsive to said attached load for generating a secondforce, and comparator means for generating an inactivating signal whenthe relationship of said forces exceeds a predetermined value, saidwinch means responding to said signal.

7. A tower crane as set forth in claim 6, wherein said comparator meansinclude a switch and a movably mounted switch actuating member, a pulleymember ber and said spring.

* t a: a

1. A tower crane comprising: a. a base structure; b. an upright columnupwardly extending from said base structure; c. a pivot on said columnnear the upper end of the column; d. an elongated boom having onelongitudinal end portion secured to said column by said pivot forangular movement in a vertical plane; e. adjusting means for varying theangular position of said boom in said plane; f. a load lifting,elongated tension member depending from the other longitudinal endportion of said boom, whereby the spacing of said pivot and of saidmember projected into a horizontal plane, varies in response to thevariation of said angular position between a minimum value and a maximumvalue; g. winch means for longitudinally raising and lowering saidtension member and a load attached thereto; and h. load limiting meansfor inactivating said winch means when a load attached to said tensionmember exceeds a predetermined value,
 1. said load limiting meansincluding means for varying said predetermined value as a function ofsaid projected spacing during said variation of the angular position ofsaid boom to satisfy the equation Lmax LOmax - k1a wherein Lmax is saidpredetermined value of said attached load, a is said projected spacing,LOmax is the value of Lmax when a amin, amin being minimum value of saidprojected spacing, and k1 is a proportionality factor and substantiallyconstant.
 2. A tower crane as set forth in claim 1, wherein the valuesof Lmax differ by not more than 10 percent from the values of Lmaxdefined by said equation when k1 is constant while a varies between amaxand amin, amax being said maximum value.
 3. A tower crane as set forthin claim 1, wherein the values of Lmax differ by not more than 5 percentfrom the values of Lmax defined by said equation when k1 is constantwhile a varies between amax and amin, amax being said maximum value. 4.A tower crane as set forth in claim 1, wherein the maximum tensilestress in said boom differs by not more than plus/minus 10 percent froma constant value while said a varies between amin and amax, amax beingsaid maximum value.
 5. A tower crane as set forth in claim 1, whereinthe maximum tensile stress in said boom differs by not more thanplus/minus 5 percent from a constant value while said a varies betweenamin and amax, amax being said maximum value.
 6. A tower crane as setforth in claim 1, wherein said load limiting means include meansresponsive to said spacing a for generating a first force, meansresponsive to said attached load for generating a second force, andcomparator means for generating an inactivating signal when therelationship of said forces exceeds a predetermined value, said winchmeans responding to said signal.
 7. A tower crane as set forth in claim6, wherein said comparator means include a switch and a movably mountedswitch actuating member, a pulley member mounted on said switchactuating member, said load lifting member being trained over saidpulley member, said pulley member and said tension member constitutingelements of said means for generating said second force, and meanstransmitting said first force to said switch actuating member to resistsaid second force.
 8. A tower crane as set forth in claim 7, whereinsaid means for generating a first force include a spring, first motiontransmitting means interposed between said boom and said spring, andsecond motion transmitting means interposed between said switchactuating member and said spring.